Thrombocytopenia is the dose-limiting toxicity of many anti-cancer and anti-HIV drugs. A standardized and optimized murine CFU-Meg assay can be used to determine the toxicity of experimental chemotherapeutic drugs to megakaryocyte progenitor cells. The CFU-Meg assay was designed to produce statistically significant, reproducible myelotoxicity data. The experiments proposed in this Phase II application will validate the use of the CFU-Meg assay for such testing. This will involve in vivo/in vitro correlations comparing toxicity in CFU-Meg assays with platelet and megakaryocyte numbers in chemotherapy-treated mice. The toxicity of chemotherapeutic agents will be further tested in CFU-Meg assays using primitive multipotential progenitors populations to identify the mechanism of early versus late onset thrombocytopenia. Purified human proteins at plasma concentrations will be added to protein-free CFU-Meg assays to understand the toxic effects of drugs under physiological conditions. Finally, human clinical data will be compared with the in vitro data to determine if the CFU-Meg assays predict the human in vivo toxicity. Such assays may be used to screen experimental and conventional drugs for potential toxicity to megakaryocyte progenitors.